Production device and production method for an optical device component having a grating structure

ABSTRACT

A production device and a production method for a grating-type optical component enabling formation of a variety types of FBGs using a single phase mask and an optical component made by the production method or production device for a grating-type optical component are provided. The method involves diffusing at least one of hydrogen or deuterium into an optical fiber and altering the refractive index of the optical fiber by irradiating the fiber with non-interfering UV lamp light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical waveguide havingphotosensitivity in the core thereof, and more specifically to aproduction device for a grating-type optical component and a productionmethod for a grating-type optical component, the properties of which areadjusted by changing the refractive index thereof using ultravioletlight (hereinafter “UV”) and to an optical component made using theproduction device for a grating-type optical component or productionmethod for a grating-type optical component.

2. Description of the Related Art

Quartz is a material having excellent optical transmission qualities andis therefore used in a variety of applications such as optical lenses orwaveguides for optical transmission or the like. The material of anoptical fiber as an optical communications line is silica based, whenproducing functional optical components such as an optical wavelengthselection filter, an optical splitter, a spectral separator orattenuator or the like in that line, there are merits, in terms ofcompatibility with optical fiber (the refractive index, core diameter,and fusion point when making a fusion connection), to produce suchfunctional components of quartz.

In an optical waveguide such as a planar light wave circuit (PLC) or anoptical fiber including photosensitive material such as Ge, phosphorusor boron added in the core, a grating is formed by irradiating UV lightof an appropriate wavelength into the optical waveguide from the sidethereof so as to alter the refractive index inside the coreperiodically, in a longitudinal direction; such gratings compriseoptical components used as an above-mentioned wavelength selectionfilter. As shown in FIG. 1A, in accordance with the desired objective,the refractive index is varied at a determined periodicity following ina longitudinal direction of the optical waveguide 100. Further, besidesa gradual, successive alteration, this alteration may be of an irregularor discontinuous periodicity.

The grating 102 formed in the core 101 of the optical fiber 100 iscalled an optical fiber grating. Normally, the optical fiber grating isclassified by the period of refractive index variation. One is longperiod grating whose period of refractive index variation is above 100μm, and the other is fiber Bragg grating (hereafter “FBG”) whose periodof refractive index variation is below a few micro meters. These areimportant optical components in the field of optical transmission.

In the description following, optical fiber refers to opticalwaveguides. In the same manner, FBG in the description refers to agrating formed inside an optical waveguide.

When the period of refractive index variation of an FBG formed in a coreis determined as Λ, wavelength λ of light reflected at the FBGsatisfying the expression

λ=2·n _(eff)·Λ₀  (1).

Here, n_(eff) is the effective refractive index of the FBG and n_(eff)is nearly equal to 1.46 at the silica-based core. As an example, if theabove expression (1) is applied to wavelength λ=1550 nm used in public(commercial) optical transmission networks, then FBG pitch Λ₀≈500 nm=0.5μm is obtained.

A conventional method for producing an FBG will now be described.Referring to FIG. 2A, firstly optical fiber 127 is disposed insidepressurized container 111. A kilometer or from several hundred to tensof meters of reeled optical fiber covered with protective coating,optical fiber covered with protective coating cut into several meterlengths, or optical fiber 127 as shown in the drawing cut into severalmeter lengths having a part of the covered protective coating materialpart 129 removed to expose the inner part are all suitable for use asthe optical fiber 127.

Next, in a condition loaded with hydrogen (H₂) or deuterium (D₂) and ina pressurized condition (e.g.: 10 NMPa-30 MNPa), high-pressure hydrogen113 or deuterium is diffused through the cladding 125 of the opticalfiber 127 reaching the core 123. This process is known as hydrogendiffusion treatment.

The object of the above hydrogen diffusion treatment is that if hydrogenor deuterium are diffused into the core 123 of the optical fiber 127then, as will be described subsequently, the photosensitivity of thecore 123 can be increased when an interfering UV laser beam is radiatedto the core 123. In other words, it is known that when imprinting anFBG, defusing hydrogen or deuterium in a core, here core 123, raises thespeed of the increase of the refractive index approximately fiftyfold incomparison to a core that has not been diffused with hydrogen ordeuterium. It is well known that in such a condition, raising thetemperature inside the pressurized container above room temperatureraises the speed of this diffusion.

When, in this hydrogen diffusion treatment, the optical fiber 127 hasbeen diffused with hydrogen or deuterium, the covering material part 129must be removed to radiate UV laser rays therein. This is because thecovering material part 129, of resin, diffused with the hydrogen,absorbs UV laser light thereby preventing the rays from reaching thecore.

Next, as highly interfering UV laser light 171 is radiated through aphase mask 173 having a specific periodicity, a fringe pattern of theinterference arises in the hydrogen diffused optical fiber core 123; thedensity of energy being higher, and thereby raising the refractiveindex, in the bright portions of this UV pattern. Usually, interferenceof diffracted light of first order through the phase mask is used, theresulting interference fringe being half the period of the phase masksuch that the period of the FBG is half the period of the phase mask. AnFBG (hydrogen diffused) 121 having an uniform period can be formed inthis way. The process itself is known as UV exposure processing.

As shown in FIG. 2C, the optical fiber with imprinted FBG is then placedin an oven 151 for a determined period of time (e.g. 12 hours) in aheated condition (120° C.) so that the hydrogen 153 or deuteriumdiffused into the optical fiber 127 is released to the outside. Thisprocess is called the hydrogen removal process. The optical fiber 117shown in FIG. 2C is an optical fiber with hydrogen removed through thehydrogen removal process, and the optical fiber covering part 119thereof is a cladding, the hydrogen in which has been removed in thesame manner.

An optical fiber having a refractive index periodically distributed at aconstant pitch Λ₀ in the core thereof inside a cladding produced in thisway, as shown in FIG. 1A, is called a uniform type FBG. In a uniformtype FBG reflection occurs at multiple points in phase in relation tosignal light of wavelength λ_(i) satisfying the above expression (1),among signal light propagating in the core. Appropriate applications canbe found in FBG for stabilization of wavelengths of laser diodes (“LD”)or FBG for Add/Drop for adding or dropping light with specificwavelengths.

Where the pitch Λ of an FBG inside a core changes successively andgradually (e.g. Λ₁-Λ_(n)), the FBG is said to be a chirped type FBG.This kind of FBG has broad bandwidth and is effective for multiplewavelengths. Appropriate applications can be found in FBG forcompensating chromatic dispersion and FBG for equalizing gain afteramplification by an optical amplifier.

SUMMARY OF THE INVENTION

The above-described conventional methods however, only allow forimprinting of FBG having the same type of pitch from one type of mask asthe periodicity of the FBG is determined by the phase mask. Thus, inorder to create FBG's having different properties, a variety ofdifferent phase masks are required leading to increased productioncosts.

With the foregoing in view, the present invention provides a method forproduction of a grating-type optical component that is superior in termsof facilitating mass production and enables formation of a variety oftypes of FBG using a single phase mask, a production device forproducing that grating-type optical component and an optical componentmade using that production method or production device for agrating-type optical component.

In a first technical aspect of the present invention the method forproduction of a grating-type optical component includes the steps of:radiating a monochromatic light of the ultraviolet region onto asilica-based optical waveguide diffused with at least one of hydrogen ordeuterium so as to alter the refractive index of the silica-basedoptical waveguide and radiating interfering light to the quartz opticalwaveguide. Further, the first aspect includes an optical componentcreated using the production method.

In a second technical aspect of the present invention the productiondevice for producing a grating-type optical component includes: a lightsource for generating a monochromatic light of the ultraviolet region, aprimary irradiation system for radiating that monochromatic light to asilica-based optical waveguide diffused with at least one of hydrogen ordeuterium and a secondary irradiation system for radiating interferinglight to the silica-based optical waveguide. Further, the second aspectincludes an optical component created using the production device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an explanatory drawing depicting an FBG for wavelengthstabilization in which incident power in relation to wavelength isillustrated in FIG. 1B as a graph, and transmission power in relation towavelength is illustrated in FIG. 1C as a graph;

FIG. 2A depicts a conventional method for the hydrogenation process;FIG. 2B depicts the UV exposure process; FIG. 2C depicts the hydrogenremoval process;

FIG. 3A illustrates process 1 related to the first embodiment of thepresent invention; FIG. 3B illustrates process 2 related to the firstembodiment of the present invention;

FIG. 4A illustrates process 3 related to the first embodiment of thepresent invention; FIG. 4B illustrates process 4 related to the firstembodiment of the present invention;

FIG. 5A illustrates changes in the refractive index of an optical fiberwhen non-interfering UV light is uniformly radiated over an entireoptical fiber; FIG. 5B illustrates changes in the refractive index of anoptical fiber as non-interfering UV light is radiated, when theamplitude of the radiated UV light is changed along the optical fiber;FIG. 5C illustrates changes in the refractive index of an optical fiberwhen part of an optical fiber is radiated with non-interfering UV lightusing amplitude mask 175;

FIG. 6A illustrates changes in the refractive index when part of anoptical fiber is radiated with non-interfering UV light using amplitudemask 175, FIG. 6B illustrates changes in the refractive index asnon-interfering UV light is radiated, when the amplitude of the radiatednon-interfering UV light is changed along the optical fiber by using theamplitude mask 175; FIG. 6C illustrates changes in the refractive indexof an optical fiber when the time of exposure to non-interfering UVlight radiated thereto is changed along the optical fiber by moving(178) the amplitude mask 175;

FIG. 7 shows the relationship between wavelength and gas used in anexcimer lamp;

FIG. 8A shows the properties of an FBG formed using a method of theprior art;

FIG. 8B shows the properties of an FBG formed using a method accordingto the present invention;

FIG. 9A shows the properties of an FBG formed using a method accordingto the present invention; FIG. 9B shows the properties of an FBG formedusing a method according to the present invention;

FIG. 10 shows the transition of central wavelength due to irradiation ofUV light by an excimer lamp;

FIG. 11A illustrates process 1 related to the second embodiment of thepresent invention; FIG. 11B illustrates process 2 related to the secondembodiment of the present invention;

FIG. 12A illustrates process 3 related to the second embodiment of thepresent invention; FIG. 12B illustrates process 4 related to the secondembodiment of the present invention;

FIG. 13 shows the transition of central wavelength in an optical fibergrating after irradiation of UV light by an excimer lamp using a methodaccording to the second embodiment;

FIG. 14 shows the transition of central wavelength due to irradiation ofUV light by an excimer lamp using a method according to the thirdembodiment;

FIG. 15A shows characteristics of transmission loss when irradiation ofUV light by an excimer lamp is not performed; FIG. 15B showscharacteristics of transmission loss when irradiation of UV light by anexcimer lamp is performed;

FIG. 16A shows the properties of a chirped type FBG formed using amethod of the prior art; FIG. 16B shows the properties of a chirped typeFBG formed using a method according to the present invention;

FIG. 17A shows UV distribution amplitude in a planar optical waveguide;FIG. 17B shows UV distribution amplitude in an optical fiber;

FIG. 18 illustrates the increase in the birefringence of an opticalfiber due to irradiation from an excimer lamp;

FIG. 19A shows an example of irradiation of one side of an optical fiberwith light from UV lamp; FIG. 19B shows an example of irradiation ofboth sides of an optical fiber with light from UV lamps; FIG. 19C showsan example of irradiation of both sides of an optical fiber with lightfrom a UV lamps; FIG. 19D shows an example of irradiation of multiplefaces of an optical fiber with light from a UV lamp where a reflectorplate is used; FIG. 19E shows an example of irradiation of multiplefaces of an optical fiber with light from a UV lamp where a curvedreflector plate is used;

FIG. 20 shows the control system (using a timer) used when altering therefractive index of an optical fiber without moving an amplitude mask;

FIG. 21 shows the control system (using an optical power meter) usedwhen altering the refractive index of an optical fiber without moving anamplitude mask;

FIG. 22 shows the control system (using an FBG) used when altering therefractive index of an optical fiber without moving an amplitude mask;

FIG. 23A shows the main flow when altering the refractive index of anoptical fiber without using an amplitude mask; FIG. 23B shows the flowof processing of the control system of FIG. 20; FIG. 23C shows the flowof processing of the control system of FIG. 21; FIG. 23D shows the flowof processing of the control system of FIG. 22;

FIG. 24 shows the control system having a timer used when altering therefractive index of an optical fiber by moving an amplitude mask;

FIG. 25 shows the control system having an optical power meter used whenaltering the refractive index of an optical fiber by moving an amplitudemask;

FIG. 26 shows the control system having an FBG used as a sensor whenaltering the refractive index of an optical fiber by moving an amplitudemask;

FIG. 27 shows the control system (amplitude mask position measuringpart) used when altering the refractive index of an optical fiber byusing an amplitude mask;

FIG. 28A shows the main flow when altering the refractive index of anoptical fiber by moving an amplitude mask; FIG. 28B shows the flow ofprocessing of the control system of FIG. 24; FIG. 28C shows the flow ofprocessing of the control system of FIG. 25; FIG. 28D shows the flow ofprocessing of the control system of FIG. 26; and FIG. 28E shows the flowof processing of the control system of FIG. 27.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference to the drawings.

First Embodiment

FIGS. 3 and 4 illustrate the production method for a grating-typeoptical component related to the first embodiment of the presentinvention.

As shown in FIG. 3A an optical fiber 27 is placed inside a pressurizedcontainer 111. Several kilometers or from several hundred to tens ofmeters of reeled optical fiber covered with protective coating, coveredoptical fiber cut into several meter lengths, or optical fiber 27 asshown in the drawing cut into several meter lengths having a part of thecovering material part 29 removed to expose the inner part, are allsuitable for use as the optical fiber 27.

Step 1 Hydrogen Diffusion Process

Next, the optical fiber is left in a hydrogen (H₂) or deuterium (D₂)loaded, pressurized condition (e.g.: 10 MPa-30 MPa) and high-pressurehydrogen 113 or deuterium is diffused through the cladding 25 of theoptical fiber 27 reaching the core 23. Where for example the diameter ofthe silica-based part is 125 μm, if pressurized at 55° C., the opticalfiber must remain in that condition for a period of five days.

If hydrogen or deuterium is diffused into the core 23 of the opticalfiber 27 then, as will be described subsequently, the photosensitivityof the core 23 can be increased when an interference capable UV laserbeam is radiated to the core 23.

Step 2 Raising Refractive Index

As shown in FIG. 3B, non-interfering UV light (hereinafter “UV light”)is then radiated to irradiation region 24 a of the optical fiber 27diffused with hydrogen or deuterium, using non-interfering UV lamp light131 that radiates over a broad area. An excimer lamp that generatesincoherent, monochromatic light is one example of a preferred lightsource for this non-interfering UV lamp light. Here, non-interferingrefers to incoherent light that does not create interference.

In step 2 above, as the UV light is irradiated, the power of thisradiation is adjusted allowing the refractive index of the optical fiber27 to be adjusted. In addition to adjusting the power of light radiatedfrom a light source, adjusting the integrated power of radiation herecan be performed by adjusting the duration thereof or by using thefollowing methods for performing adjustment of the refractive index.

For example, as shown in FIG. 5A, raising the base line level ofrefractive index (refractive index profiles) 550, of the entire opticalfiber by radiating a fixed power of non-interfering UV light 501 overthe entire optical fiber 507 can be used.

Further, as shown in FIG. 5B, a changing of the refractive index in alongitudinal direction of an optical fiber 507 in which the degree ofincrease in the refractive index is adjusted in different portions ofthe optical fiber 507 by radiating varying quantities of non-interferingUV light 502 to those portions can also be used. This raising therefractive index 551 results in a transition in the refractive index asindicated by the sloping line in FIG. 5B.

Again, as shown in FIG. 5C, there can be a raising of the refractiveindex 552 applied to only a part of an optical fiber, achieved byradiating only specific portions of the optical fiber with UV light 503by using an amplitude mask 175 having slits disposed in parts thereof orusing two masks 175 to block part of the UV light radiated.

Further, as shown in FIG. 6A, there can be a raising of the refractiveindex 553 in only a part of one side of an optical fiber 507 by using anamplitude mask 175 having slits that maintains a constant power of UVlight 504.

As shown in FIG. 6B, by radiating UV light 505 via an amplitude mask 175in which the rate of permeation of light is disparate in differentplaces, the degree of the increase in the refractive index can beadjusted in relation to a location, thereby enabling the refractiveindex to be altered following a longitudinal direction of the opticalfiber 507. This raising of the refractive index from the bottom up 554can be adjusted with changes in the properties of the amplitude of theadjusting mask used, as illustrated by the sloping line in the exampleon FIG. 6B.

Moreover, as shown in FIG. 6C, the refractive index can be alteredfollowing a longitudinal direction of the optical fiber 507 by adjustingthe duration of exposure of the UV light through adjusting the speed ofmovement of the amplitude mask 175 having slits, that maintains aconstant power of UV light 506 radiated, as that mask is moved. Thisraising of the refractive index from the bottom up 555, can be adjustedby controlling the movement 178 of the amplitude mask 175, asillustrated by the sloping line in the example of FIG. 6C. FIG. 3Billustrates the case where the method of FIG. 6C is used.

Although the required wavelengths of UV light used in order to obtainthe above described changes in the refractive index is dependent to somedegree on the sensitizing material added, normally wavelengths of below280 nm are required. This is because it is difficult to obtain thedesired refractive index with extremely low photosensitivity. On theother hand, at below 150 nm, there is an extremely high rate of lightabsorption by pure quartz having no sensitizing material, such thatlight cannot penetrate; accordingly the wavelength of radiated lightmust be above 150 nm.

For this reason light sources generating light of wavelengths in theregion between 150 and 280 nm from among those excimer lamps shown inFIG. 7 can be used. Among those the 222 nm lamp using KrCl and the 172nm lamp using Xe are the most convenient, as they can be economicallyobtained and provide stable lamp functions because they are used forcleaning purposes on liquid crystal panels and the like.

An excimer lamp, as opposed to an excimer laser, can be used for a UVlight source. Whereas the power intensity of the laser is 1000 timesthat of the lamp, the lamp can radiate light over a broader area, 500×80mm. A small type excimer lamp can be as small as 20 mm×18 mmφ and isapproximately one tenth the cost of a laser light source, while lightradiated from an excimer lamp for a long period is not highly conduciveto deterioration in the mechanical strength of silica-based material ofan optical fiber, and the 100V or 200V power supply used by the lampmakes for compatibility with ordinary commercial power supply voltage.Further, as the power density of an excimer lamp is low compared to thatof a laser lamp, light from the excimer lamp does not cause damage to anoptical waveguide during the exposure period.

Step 3 Hydrogen Removal Process

Referring to FIG. 4A, the optical fiber 27 is placed inside an oven 151and left therein for a set duration (12 hours), remaining in a heatedcondition (120° C.), enabling the hydrogen 153 or deuterium diffused inthe optical fiber 27 to be released to the outside. Numeric 17 in FIG.4A, indicates the optical fiber with hydrogen removed therefrom. Numeric15 in FIG. 4B is the cladding with hydrogen removed and numeric 13 inthat figure is the core with hydrogen removed.

As due to this step, the density of remaining hydrogen or deuteriuminside the optical fiber 17 is lowered sufficiently, even where there isa long period of standby in which the optical fiber is kept before thesubsequent step, basically no changes occur in the properties of theoptical fiber during that time. Even with the hydrogen or deuterium thusremoved however, photosensitivity to UV laser light during the formationof the FBG is still maintained. It is a characteristic of this inventionthat the photosensitivity of the core does not decrease after removal ofthe hydrogen or deuterium to the extent that occurs when technology ofthe present invention is used.

Silica-based material that has undergone the above-described steps hasincreased photosensitivity and the deterioration over time of thatphotosensitivity is small, thereby enabling creation of an opticalwaveguide type optical component.

Further, in addition to each of the above steps, a step 4 enablesformation of an FBG by radiating interfering UV light to the opticalfiber. Normally, a light source having an interfering effect such as alaser or the like that utilizes changes in amplitude through theinterference of light, is used in order to change the refractive indexof a core. The second harmonic of an argon ion laser or an excimer lasercan be used to provide such a light source.

An example of a short period FBG formed using a phase mask 173, as shownin FIG. 4B, provides an example of the first embodiment of the FBGproduction method of the present invention. Here, as interfering UVlight 171 is radiated through a phase mask 173 having a fixedperiodicity, a fringe pattern arises in the core 13 (having no hydrogen)of the optical fiber, the refractive index of the core being raised atpoints exposed to the high brightness of the UV. FBG 11 (having nohydrogen) having a fixed period can be formed in this way. In additionto this method of using a phase mask as a method of forming an FBG, atwin beam interference method can also be used.

Generally, as shown in FIG. 8A, the reflected center wavelength λdetermined from the pitch Λ₀ of a uniform type FBG formed using thismethod, results in λ=2N₁Λ₀ when expression (1) is applied. N₁ is theeffective refractive index in the FBG region.

An FBG formed by step 1 of the method of this invention has therefractive index raised by UV lamp light, such that the reflectioncentral wavelength is changed, becoming longer. Where the refractiveindex of an entire core of an optical fiber is raised, 556 as shown inFIG. 8B, the reflected center wavelength is λ₂=2N₂Λ₀, and there is ashift toward longer wavelengths. Here, N₂ is the effective refractiveindex of FBG regions having raised refractive index.

FIG. 9A shows the case where four levels of refractive index are formedusing a graded form of raising of refractive index. These four levels ofeffective refractive index of an FBG are termed, respectively, N₁, N₂,N₃ and N₄ (N₁<N₂<N₃<N₄). Four values exist for the reflection centralwavelength, respectively, λ₁=2N₁Λ₀, λ₂=2N₂Λ₀, λ₃=2N₃Λ₀, λ₄=2N₄Λ₀(λ₁<λ₂<λ₃<λ₄). The FBG with four refractive indexes is equivalent to acombination of four FBGs, each has one refractive index and a differentperiod of those four ones.

Where the sloping raising of refractive index 558 is formed, as shown inFIG. 9B, as the refractive index gradually increases in a longitudinaldirection of the optical fiber, the reflection central wavelength alsogradually changes toward the longer wavelength side, such thatreflection arises in some wavelength bandwidths, resulting in theappearance of transmission loss in those bandwidths.

The results of an FBG of this embodiment experimentally produced willnow be described. The optical fiber of the optical waveguide was singlemode optical fiber, with approximately 3.5 Wt % GeO₂ added, the corediameter being approximately 10 μm and the difference in a specificrefractive index between the core and cladding being 0.35%. This opticalfiber was placed in a 55° C., 10 MPa hydrogen atmosphere and left therefor one week to allow the hydrogen to penetrate through to the centerpart of the optical fiber. The UV-curable resin providing a protectivecoating around the quartz optical fiber does not allow ultra violetlight to penetrate, so that the resin was removed in parts to expose thequartz.

After the exposed quartz parts were irradiated for a fixed duration withultra violet light from an excimer lamp having power density of 15mW/cm², hydrogen removal processing was performed for 12 hours at 120°C. The wavelength radiated from the excimer lamp was 172 nm. Thereafter,using the phase mask method, a FBG with a reflection central wavelengthof 1550 nm was formed on the lamp irradiated portions using the secondharmonic (wavelength 244 nm) from an argon ion laser. At this time auniform mask having equal periodicity was used for the phase mask suchthat each period of the periodicity of the FBG was equal. The length ofthe FBG region was 3 mm. The results obtained by measuring the changesof the central wavelength of the FBG are shown in FIG. 10. Thehorizontal axis in FIG. 10 shows time duration of exposure to theexcimer lamp and the vertical axis, the degree of change in centralwavelength of the FBG, taking the properties of the FBG without exposureto an excimer lamp as the base of measurement.

As shown in FIG. 10, as the length of time of exposure to the excimerlamp increases the refractive index rises, and notwithstanding the factthat the periodicity of the FBG is constant, changes in the centralwavelength were confirmed. This relationship between the time durationof radiation exposure and changes in central wavelength sits very wellabove a plain curved line, and as there is a one-to-one relationshipbetween change in refractive index and duration of radiation exposure,the desired change in refractive index can be obtained simply bycontrolling this time of exposure, thereby confirming that it ispossible to create a grating having the desired central wavelength.

The difference between refractive index when non-interfering UV lamplight (excimer lamp light) is radiated to one side of an opticalwaveguide and radiated to multiple faces of an optical waveguide will beconsidered.

FIG. 17A shows a core 702 and a cladding 703 disposed on a planaroptical waveguide substrate 700. Due to the structure of the planaroptical waveguide the face of the radiation is restricted to one side ofthe structure. That is to say, the side and lower faces of the opticalwaveguide are a thick substrate, such that irradiation of the opticalwaveguide from the surrounding area other than from the upper surface isdifficult. Accordingly there is a substantial difference 6 of the lightamplitude in the inside and the upper surface of the optical waveguideclose to the lamp light 701, thus, it can be estimated that the doublerefraction increases.

On the other hand, in the case illustrated in FIG. 17B radiated lighteasily reaches from all around the optical waveguide and even if forexample, UV lamp 711 irradiates only one side of the optical waveguide,the core 705 can still be irradiated due to the reflection and dispersalof the UV light 712 and 713 occurring within the optical fiber andcoming from the material surrounding the fiber. Accordingly as thedifference 6 of the light amplitude in the inside and the upper surfaceof the optical waveguide close to the lamp light is relatively small, itcan be estimated that the double refraction is small. Further, as thesupplementary UV light 712 and 713 is radiated from around the opticalfiber, double refraction is lowered further.

FIG. 18 illustrates the relationship of double refraction and durationof radiation time when UV lamp light irradiated one side of the opticalfiber and multiple faces of the optical fiber. The duration of radiationfrom the excimer lamp is plotted on the horizontal axis in that figureand values for double refraction are plotted on the vertical axis.

FIGS. 19A-19E depict methods for realizing the above-describedirradiation of different aspects of an optical waveguide with Lw lamplight.

FIG. 19A illustrates the case where UV lamp light 801 irradiates opticalfiber 811 from one side. FIGS. 19B to 19E illustrate radiation of UVlamp light to multiple faces of the optical waveguide. In FIG. 19B, thecase of having two lamps 802 and 803 juxtaposed, irradiating both sidesof optical fiber 811 is illustrated. FIG. 19C shows the case where twoexcimer lamps 804 and 805 are arranged on opposite sides of opticalfiber 811, so as to radiate to both sides thereof. FIG. 19D shows areflection plate 820 disposed opposing excimer lamp 806, such that aslight radiated from the single excimer lamp is reflected and dispersedfrom this plate, the optical fiber 811 is irradiated with light frommultiple directions. Finally, FIG. 19E illustrates that the shapecross-sectionally of a curved reflection plate 821 may be elliptical.Further, the reflection plate may be an ellipsoid having the opticalfiber 811 and the excimer lamp 807 disposed respectively in twodifferent focal points therein.

The maximum value of double refraction of an FBG formed according to theabove method is, in the case of irradiation from one side, 0.7×10⁻⁴.Further, it was confirmed that double refraction can be below 0.1×10⁻⁴where radiation from the excimer lamp is directed to multiple surfacesof the optical waveguide. Accordingly, polarization dependent loss (PDL)and polarization mode dispersion (PMD) arising due to double refractioncan be reduced.

Second Embodiment

FIGS. 11 and 12 illustrate a method for producing a grating-type opticalcomponent related to the second embodiment of the present invention. Asstep 1 of FIG. 11A is the same as step 1 of the first embodimentdepicted in FIG. 3A a description of step 1 of FIG. 11A is omitted here.

As shown in FIG. 11B, in step 2 of this embodiment interfering UV laserlight 171 is radiated, via phase mask 173, to optical fiber 27 to formFBG 21 with hydrogen diffused on the optical waveguide core part 23.

According to this second embodiment, the optical fiber is single modehaving a specific refractive index difference of 0.35%, withapproximately 3.5 Wt % GeO₂ added, the core diameter being approximately10 μm.

Descriptions of step 3 of the second embodiment shown in FIG. 12A and ofstep 4 of that embodiment shown in FIG. 12B are omitted here, thosesteps being the same respectively as step 2 of the first embodimentshown in FIG. 3B and step 3 of the first embodiment shown in FIG. 4A.

According to this second embodiment, the order of irradiation using theexcimer lamp and the laser light to form an FBG, performed under thesame conditions as described with respect to the first embodiment, waschanged. Moreover, removal of the hydrogen was performed as the laststep. That is to say, the steps hydrogen diffusion, irradiation withlaser light, irradiation from an excimer lamp and hydrogen removal wereperformed in that order and changes in the central wavelength of thelaser light were confirmed. The results are shown in FIG. 13. It wasevident that changes in the central wavelength of reflected light fromthe FBG were the same as those apparent in the case of the firstembodiment even where irradiation with the excimer lamp was performed atthis stage among the order of the steps, thus confirming that theresults for central wavelength of reflected light from the FBG changeswere the same regardless of the order in which the excimer lampirradiation and FBG formation steps were performed.

The maximum value of double refraction of an FBG created according tothe above method is, in the case of radiation with an excimer lamp fromone side, 0.7×10⁻⁴. Further, in the same manner as applied with respectto the first embodiment it was confirmed that double refraction can bebelow 0.1×10⁻⁴ where radiation from the excimer lamp is directed tomultiple surfaces of the optical waveguide. Because these values are thesame as those obtained using the FBG formed in accordance with themethod of the first embodiment, it was confirmed that the same resultswere obtained with respect to the properties of double refractionregardless of whether the excimer lamp irradiation step or the FBGformation step is performed first. Accordingly, PDL and PMD arising dueto double refraction can be reduced.

Third Embodiment

A production method for a grating-type optical component related to athird embodiment of the present invention will now be described.According to this third embodiment, an experimental single mode opticalfiber having a specific refractive index difference of 0.85%, withapproximately 8.5 Wt % GeO₂ added and a core diameter of approximately 4μm was produced.

In step 1, this optical fiber was placed in a 55° C., 10 MPa hydrogenatmosphere and left there for one week to allow the hydrogen topenetrate through to the core of the optical fiber. The UV-curableresin, providing a protective coating around the quartz optical fiber,does not allow ultra violet light to penetrate, so this was removed inparts to expose the quartz.

At step 2, non-interfering UV lamp light 131 was uniformly radiated overthe entire optical fiber as shown in FIG. 12A, raising the refractiveindex as shown in FIG. 5A. However, at this step a amplitude mask wasnot used. Further, the amplitude of the UV lamp light providing thelight source was 10 mW/cm².

At step 3, after the exposed quartz parts were irradiated for a fixedduration with ultra violet light from an excimer lamp having powerdensity of 10 mW/cm², hydrogen removal processing was performed for 12hours at 120° C. The wavelength radiated from the excimer lamp was 172nm. Thereafter, using the phase mask method, a reflection centralwavelength 1550 nm FBG was formed on the lamp irradiated portions usingthe second harmonic (wavelength 244 nm) from an argon ion laser. At thistime a uniform mask having equal periodicity was used for the phase masksuch that each period of the periodicity of the FBG was equal. Thelength of the FBG region was 3 mm.

FIG. 14 shows the results obtained after the above steps were performed.The horizontal axis in FIG. 14 shows time duration of exposure to theexcimer lamp and the vertical axis, the degree of change in centralwavelength of the FBG, taking the properties of a sample not exposed toexcimer lamp irradiation as the base of measurement. It is apparent thatas the length of time of exposure to the excimer lamp increases, therefractive index rises, and notwithstanding the fact that theperiodicity of the FBG is constant, changes in the central wavelengthwere confirmed. This relationship between the time duration of radiationexposure and changes in central wavelength sits very well above a plaincurved line, and as there is a one-to-one relationship between change inrefractive index and duration of radiation exposure, the desired changein refractive index can be obtained simply by controlling this time ofexposure, thereby confirming that it is possible to form a gratinghaving the desired central wavelength.

The trend of increase in the degree of change in central wavelength (acurve shaped line) is largely the same in comparison to the resultsobtained with respect to the first embodiment. However, the absolutevalues for exposure time to the light and central wavelength change aredifferent. This is because the amount of added photosensitive material(here, Ge) as well as the structure of the optical fiber were differentand also because the respective amplitudes of the excimer lamp radiationwere different.

Generally, the speed of change in refractive index increases as theamplitude of light radiated from an excimer lamp increases and themaximum degree and speed of change in refractive index increases in linewith the amount of photosensitive material added. As shown with respectto the first and second embodiments however, even where the amount ofphotosensitive material added and the optical fiber structures differ, arelationship of the power of excimer lamp radiation and the degree ofcentral wavelength change, in other words, refractive index change,exhibits a relationship conforming to the same curve shaped line. Thatis to say, even where the amplitude of excimer lamp radiation or theamount of added photosensitive material are changed, it is possible forthe desired properties to be easily obtained from the relationshipbetween radiation time and refractive index change in accordance withthose conditions. Further, those desired properties can be readilyobtained in a short time by setting the appropriate structure for theoptical fiber, such as the amount of added photosensitive material, andamplitude of excimer lamp radiation.

The maximum value for double refraction of an FBG created according tothe above method is 0.8×10⁻⁴ when only one side of the optical fiber isexposed to excimer lamp radiation. Further, double refraction can bebrought below 0.1×10⁻⁴ by irradiating multiple surfaces of the opticalfiber using the excimer lamp as described. Accordingly PDL and PMDarising due to double refraction can be reduced.

After the above described step 3 of this third embodiment, step 4 forhydrogen removal is performed; however, this step 4 is the same as thestep 4 of the second embodiment of this invention shown in FIG. 12B.Therefore, a description of this step 4 is omitted here.

Fourth Embodiment

A production method for a grating-type optical component according to afourth embodiment of the present invention will now be described. Step 1of this fourth embodiment is the same has step 1 of the first embodimentshown in FIG. 3A therefore a description of this step one is omittedhere.

As shown in FIG. 6C, at step 2 of this fourth embodiment the duration ofblocking non-interfering UV lamp light 131 was successively altered bymoving the amplitude mask 175 at a predetermined speed, thereby changingthe duration of exposure to light of the optical fiber core. As shown inFIG. 6C, a sloping line was obtained for refractive index under theseconditions. Further, a single periodicity phase mask was used producinga substantially chirped FBG. The length of the change in refractiveindex represented by this refractive index sloping line, that is alength of the entire length of the FBG, was 100 mm.

The experimental optical fiber used for this fourth embodiment was asingle mode optical fiber having a specific refractive index differenceof 0.35%, with approximately 3.5 Wt % GeO₂ added and a core diameter ofapproximately 10 μm. A light source having UV lamp light of a power of15 mW/cm² was used.

For this embodiment the required exposure time to UV light in alengthwise direction of the optical fiber was obtained as a linearfunction of a position in a lengthwise direction of the optical fiber atan accuracy confining wavelength deviation to a range of 2 nm at thelocation of the maximum and 0 nm at the location of the minimum, basedon the results of a second embodiment. Further, based on these resultsthe desired movement of the amplitude mask 175 was obtained and for step2, the amplitude mask 175 was moved based on those results.

At step 3, hydrogen was removed from the optical fiber. This step of theprocess employed here was the same as that employed in step 3 of thefirst embodiment as shown in FIG. 4A therefore a description of thisstep is omitted here.

Thereafter, at step 4, in the region for refractive index change, a 100mm FBG was formed by exposure to light using the phase mask method usinga uniform mask. The laser used for this light exposure was an argon ionlaser radiating light of the second harmonic (wavelength 244 nm). FIG.15B shows the characteristics of transmission loss of the FBG obtainedby this process. FIG. 15A shows the transmission spectral after exposureto the same light without irradiation with an excimer lamp beingperformed. As is apparent from FIG. 15B, it was confirmed that radiationwith the excimer lamp enables formation of an FBG having a broad band,and even where a uniform mask is used a substantially chirped FBG can beformed. This kind of chirped FBG can be applied for a chromaticdispersion compensator or the like.

The refractive index of the FBG formed in this way was 0.3×10⁻⁴ when oneside of the optical fiber was exposed to the radiation using the excimerlamp. Further, it was confirmed that double refraction can be below0.1×10⁻⁴ where radiation from the excimer lamp is directed to multiplesurfaces of the optical fiber. Accordingly, PDL and PMD arising due todouble refraction can be reduced.

The FBG produced for this experiment using a uniform type phase maskhaving single periodicity has the reflection properties illustrated inFIG. 16B. In portions of the FBG of substantial refractive index changethrough exposure to UV light irradiation was largest, reflection centralwavelength λ_(n) was longest, as the refractive index becomessuccessively, gradually lower, the reflection central wavelength alsobecomes shorter in proportion thereto, such that in the FBG portions atthe lowest point of the sloping line indicating refractive index thelowest value for reflection central wavelength is λ₁. As shown in FIG.16A, this is the same result as a chirped type FBG having successivelyaltered FBG pitch using methods of the prior art.

As can be seen from the description of the above embodiments, it isirrelevant whether the step for changing refractive index using UV lightirradiated from a non-interfering light source or the step for creatingthe FBG using exposure to interfering light is performed first.

Control Systems

Control systems for the present invention are used for controllingoperations as the refractive index of an optical fiber is altered.

These control systems can be classified as refractive index adjustmentmethods as shown in FIGS. 5A and 6B. That is to say, a method that doesnot use the amplitude mask or a method in which the amplitude mask isnot moved by using a fixed type amplitude mask having a plurality ofslits of different widths, or an adjustment method as shown in FIG. 6Cthat involves moving the amplitude mask.

First Control System

FIG. 23A is a main flow chart showing operations when an amplitude maskis not used or when the amplitude mask is not moved.

Referring to FIG. 23A, after the control processes commence (S251)firstly the excimer lamp comes on (S252) and control process A foradjusting refractive index is performed (S253). When that step iscompleted the excimer lamp goes off (S254) and the series of processesis complete (S255). As the details of the process A differs inaccordance with the structure of the control system, these are describedfollowing.

FIG. 20 provides an example of a control system in which an amplitudemask is not used or in which an amplitude mask is not moved for theadjusting of refractive index.

This control system comprises an optical fiber 17 or 27, a lamp driverpart 31 for radiating non-interfering UV lamp light 131 from an excimerlamp 30, a control part 32 for controlling the on/off conditions of theexcimer lamp 30 and a timer 33 that operates as a determining means fordetermining the degree of refractive index change.

When this control system is utilized, the required duration ofirradiation time in order to obtain the desired refractive index isfirst estimated, and once the time elapsed from commencement ofirradiation of the light is detected, by the timer 33, as havingexceeded the set duration of irradiation time, radiation of the light isstopped by the control part 32.

FIG. 23B is a flow chart depicting process A of the above controlsystem. As shown in FIG. 23B, as lighting of the excimer lamp 30 isconfirmed, operation of a clock commences from the timer 33 (S2531 a)and once the duration of time elapsed exceeds the set duration ofradiation time (S2532 a: No), the process is completed (S 2533 a) andthe excimer lamp 30 stops lighting (S254).

Further, the structure of this kind of control system may be a structurehaving an optical power meter as shown in FIG. 21. When this kind ofcontrol system is used the required power of irradiation (the integratedpower of irradiated light) in order to obtain the desired refractiveindex is first estimated and once the estimated power of light fromcommencement of radiation of the light is detected, by the optical powermeter 40, as having reached the power of light set, radiation of thelight is stopped by the control part 32.

FIG. 23C is a flow chart depicting process A of the above controlsystem. As shown in that FIG. 23C, as lighting of the excimer lamp 30 isconfirmed, measurement of light by an optical power meter 40 commences(S2531 b) and once the power of light irradiated exceeds the setestimated power (S2532 b: No), the process is completed (S2533 b) andthe excimer lamp 30 stops lighting (S254).

A control system may be of a structure having a measuring part forcentral wavelength measurement of reflected light from an optical fibergrating or the like. This kind of control system is shown in FIG. 22.

An optical fiber grating 38 for a central wavelength detector is formedon an optical fiber 39. This optical fiber 39 can be an optical fiberwhich has the same photosensitivity as the optical fiber 17, 27 or anoptical fiber the degree of change of the refractive index is known. Thecentral wavelength measurement part 34 measures the change of centralwavelength of reflected light from the optical fiber grating due to theradiated UV lamp light 131.

With this structure of control system, radiation of the UV light can bestopped at the point in time at which the desired refractive indexchange is achieved because the change of refractive index in the opticalfiber 17 or the optical fiber 27 can be estimated in real-time from thechange of central wavelength arising as the light is radiated to theoptical fiber grating 38.

Further, a structure can also be configured in which the degree ofchange of central wavelength indicating the desired change in refractiveindex is set in advance and radiation of the light is stopped when thisdegree of change in central wavelength is reached. A process flow chartfor this kind of system is shown in FIG. 23D.

As shown in FIG. 23D, as lighting of the excimer lamp 30 is confirmed,central wavelength measurement commences from a central wavelengthmeasurement part 34 (S2531 c), and once the degree of measured centralwavelength change, exceeds the set degree of wavelength change (S2532 c:No), the process is completed (S2533 c) and the excimer lamp 30 stopslighting (S254).

Second Control System

FIG. 24 shows an example of a control system for adjusting/tailoringrefractive index profiles when a amplitude mask is moved (178) or whenthe amplitude mask is not moved.

This control system has, added to the system of FIG. 20, an amplitudemask driver part 36 for driving the amplitude mask, a control part 37for controlling the amplitude mask driver part 36 and a personalcomputer (PC) 35, while the functions of the other components of thestructure are the same as those of the control system described withrespect to FIG. 20. The PC 35 receives signals from the timer 33 andoperates the control parts 32 and 37 based on those signals.

FIG. 28A is a main flow chart showing the operations of the controlsystem when the amplitude mask is moved. Referring to FIG. 28A, afterthe control processes commence (S301) firstly the amplitude masktransitions to the starting point (the initial position) as necessary(S302) and once this is confirmed the excimer lamp comes on (S303) andcontrol process B is implemented (S304) for adjusting refractive index.When that step is completed the excimer lamp goes off (S305) and theseries of processes is complete (S306). The details of the process Bdiffers in accordance with the structure of the control system.

FIG. 28B is a flow chart depicting process B of the above controlsystem. As shown in FIG. 28B, as lighting of the excimer lamp 30 isconfirmed, processes commence (S3041 a), operation of a clock commencingfrom the timer 33 (S3042 a) while movement 178 of the amplitude mask 175commences (S3043 a). Where necessary, speed v(t) is changed inaccordance with time elapsed, and once the duration of time elapsedexceeds the set duration of irradiation time (S3044 a: No), the processis completed (S3045 a) and the excimer lamp 30 stops lighting (S305).

FIG. 25 shows a control system in which an optical power meter 40 isinstalled instead of the timer 33 shown in FIG. 24. A PC 35 receivessignals from the optical power meter 40 and operates the control parts32 and 37 based on these signals; this being the only point ofdifference between the control system shown in FIG. 25 and the controlsystem shown in FIG. 24 and in all other respects the functions of thecomponents comprising the structure of this control system are the sameas those of the control system shown in FIG. 24.

FIG. 28C is a flow chart depicting process B of the control system shownin FIG. 25. As shown in FIG. 28C, as lighting of the excimer lamp 30 isconfirmed, processes commence (S3041 b), measurement of light from theoptical power meter 40 commences (S3042 b) while movement of theamplitude mask 175 begins (S3043 b). Where necessary, speed v(t) ischanged in accordance with an integrated irradiated power of light, andonce the integrated power of light irradiated exceeds the set integratedirradiated power of light (S3044 a: No), the process is completed (S3045b) and the excimer lamp 30 stops lighting (S305).

FIG. 26 shows a control system in which instead of the optical powermeter 40 shown in FIG. 25, an optical fiber 39, optical fiber grating 38and central wavelength measuring part 34 are installed. A PC 35 receivessignals from the central wavelength measuring part 34 and operates thecontrol parts 32 and 37 based on these signals; these being the onlypoints of difference between the control system shown in FIG. 26 and thecontrol system shown in FIG. 25 and in all other respects the functionsof the components comprising the structure of this control system arethe same as those of the control system shown in FIG. 25.

FIG. 28D is a flow chart depicting process B of the control system shownin FIG. 26. As shown in FIG. 28D, as lighting of the excimer lamp 30 isconfirmed, processes commence (S3041 c), central wavelength measurementcommencing from the central wavelength measurement part 34 (S3042 c),while movement of the amplitude mask 175 commences (S3043 c). Wherenecessary, speed v(t) is changed in accordance with central wavelengthand once the measured degree of change of central wavelength exceeds theset degree of change of central wavelength (S3044 c: No), the process iscompleted (S3045 c) and the excimer lamp 30 stops lighting (S305).

FIG. 27 shows a control system in which an amplitude mask positionmeasuring part 42 for measuring the position of an amplitude mask 175 isinstalled instead of the central wavelength measuring part 34 shown inFIG. 26. A PC 35 receives signals from the amplitude mask positionmeasuring part 42 and operates the control parts 32 and 37 based onthese signals; these being the only points of difference between thecontrol system shown in FIG. 27 and the control system shown in FIG. 26and in all other respects the functions of the components comprising thestructure of this control system are the same as those of the controlsystem shown in FIG. 26.

FIG. 28E is a flow chart depicting the process B of the control systemshown in FIG. 27. As shown in FIG. 28E, as lighting of the excimer lamp30 is confirmed, processes commence (S3041 d), measurement of theposition of the amplitude mask commencing from the amplitude maskposition measuring part 42 (S3042 d), while movement of the amplitudemask 175 commences (S3043 d). Where necessary, speed v(t) is changed inaccordance with the position of the mask and once the mask 175 exceedsthe prescribed position for termination of the process (S3044 d: No),the process is completed (S3045 d) and the excimer lamp 30 stopslighting (S305).

Effects of the Invention

According to the present invention an non-interfering light can bereadily altered by changing the refractive index of a silica-basedwaveguide, enabling a substantial number of various FBG to beeconomically created using a single phase mask. Further, asnon-interfering light of an excimer lamp is used to provide amonochromatic light source, the required light source can be easilyobtained.

According to the present invention the desired FBG can be formed bycontrolling and altering the method of irradiation of the monochromaticlight.

This application claims benefit of priority under 35 USC § 119 toJapanese Patent Application No. 2003-206061, filed on Aug. 5, 2003, theentire contents of which are incorporated by reference herein. Althoughthe invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the teachings. The scope of the invention is defined withreference to the following claims.

1. An optical component comprising: a silica-based optical mediumcomprising a first section having a first refractive index, and secondsection having a periodic refractive index variation, wherein a smallestrefractive index in said second section is equal to or greater than saidfirst refractive index, and wherein a base line level of refractiveindex of the second section is a gradient which gradually andcontinuously increases in a direction of the periodic refractive indexvariation.
 2. The optical component of claim 1, wherein saidsilica-based optical medium is an optical fiber, said second sectionbeing a fiber Bragg grating (FBG).
 3. The optical component of claim 2,wherein a maximum value of double refraction of the FBG is 0.7×10⁻⁴. 4.The optical component of claim 2, wherein double refraction of the FBGis below 0.1×10⁻⁴.